Conventional semiconductor wafer singulation is done using mechanical dicing saws. Next generation semiconductor devices, however, will use new materials, such as low dielectric constant (low-k) interlayer dielectric layers (ILDs) that may be incapable of withstanding the sheer forces exerted during the wafer sawing process. Lasers, either alone or in combination with saws, are being considered as alternatives to conventional sawing. However, the integration of laser technology to perform die singulation is not without its challenges.
Conventional lasers (traditionally focused lasers having long pulse durations, such as nanosecond lasers) are currently incapable of producing high-quality cuts through a semiconductor wafer. This is because the laser's fluence is not intense enough to ablate completely through the entire thickness of the wafer. Loosening the laser's focus geometry to increase the laser's depth-of-focus does not solve the problem because enlarging the laser's depth-of-focus also reduces the laser's fluence and ablation capabilities. Conventional focusing does not improve dicing/milling capabilities because even tight focusing cannot produce the amount of fluence needed to ablate entirely through the thickness of the substrate. Increasing overall laser power to compensate for this limitation generates heat and compromises the quality of the laser cut. Heat produces thermal effects and mechanical damage. Both of which can impact a semiconductor device performance and yield.
A number of approaches are being investigated to overcome these problems. One includes using a combination of laser scribing and mechanical dicing to singulate the wafer. This approach uses multiple passes of a laser to remove scribe line (street region) material prior to saw singulating. A disadvantage of using this approach is that two different processes must be combined to singulate the wafer. The combination is more complicated, costly, and slower than using a single laser or sawing process.
Another approach uses a galvanometer steered laser to perform a series laser beam raster scans at varying focal depths to perform singulation. Major disadvantages of this process include its low throughput (low number of wafers-per-hour) and inaccurate laser beam placement (10 microns or more beam pointing instability). Unless process throughputs can be increased, this approach will probably not be considered a viable dicing alternative by high volume semiconductor manufacturers.
Yet another approach projects a laser through stream of water (water jet) onto the wafer. The water jet functions as a waveguide for the laser and produces a collimated high intensity beam. This approach's advantages include its high-throughput singulation capabilities since the laser beam stays collimated throughout whole thickness of a semiconductor wafer, and its inherent ability to cool the wafer surface using the stream of water. Disadvantages of using this process include the physical force of the water jet and the water cavitation pressure wave that forms in the trench during processing. Both of which can result in increased defect densities as well as delamination and cracking of layers formed on the wafer.